The goal of this study is to evaluate infant thermoregulatory capabilities when presented with environmental and endogenous thermal challenges. Thus far, we have studied groups of both healthy term infants and siblings of SIDS infants during daytime and evening naps to evaluate their Evaporative Water Loss (EWL) rates. We have also studied a group of infants during the 24 hour period after they received their routine DTP immunizations. EWL occurred in both REM and NREM sleep states in both groups. The magnitude of EWL was variable between subjects. Some subjects exhibited EWL behavior suggestive of different temperature thresholds for water loss in different sleep states. There was no apparent affect of the subject's age on EWL responses during wake, REM, NREM. Individual mean EWL rates generally showed a higher rate during REM. There were no obvious differences in the EWL response of siblings of SIDS compared to control infants. No prolonged apnea (respiratory rate less than or equal to 15 sec.) was seen during warmed or non-warmed conditions.